Band saws are well-known in the art. Typically, such band saws include an endless band with a sawtooth formation along one edge thereof. The band is mounted for movement in a vertical plane by upper and lower spindles. Normally, the lower spindle is driven by a belt which in turn is driven from a powered drive shaft. The drive shaft and the shaft mounting the lower spindle are preferably provided with sprockets of different diameters to bring about plural speeds of operation, such as a high speed and a low speed. The speeds are selected by manually training the drive belt around selected sprockets on the drive shaft and the lower spindle.
A belt tensioning device is preferably provided to maintain the drive belt taut at all times and irrespective of the sprockets engaged by the drive belt. Prior art tensioning devices are awkward to work with as they must be held out of engagement with the belt during a belt changing operation as a different speed of the band saw is desired.
Generally speaking, belt tensioning devices of various forms are well known in the art. For example, U.S. Pat. Nos. 2,499,287 entitled, Power Transmission Mechanism; 3,057,218 entitled, Drive Assembly; 3,762,229 entitled, Variable Power Ratio Device; and 4,917,654 entitled, Belt Tensioning Arrangement, each disclose a means of providing automatic tensioning of a drive belt as it is stretched during use. The tensioning devices disclosed in the foregoing U.S. Pats., however, are designed to be always in engagement with the drive belt.
U.S. Pat. Nos. 1,423,172 entitled, Ratchet Clutch For Drive Mechanism, and 4,530,682 entitled, Tensioning Arrangement With a Damping Device For a Belt Drive, disclose means for disengaging belt tightening devices to enable the changing of the drive belt. In U.S. Pat. No. 1,423,172, a spring biased lever is used to engage and disengage the tensioning means from the drive belt. A spring biases the lever toward the disengaged position. Thus, the operator must apply continuous pressure to the lever to maintain engagement of the tensioner with the drive belt. In U.S. Pat. No. 4,530,682, the tensioner is disengaged by loosening a screw or bolt which maintains the tensioning device in a position to engage the drive belt.
The present invention overcomes the inefficiencies of the devices known and disclosed in the prior art. The present invention by use of an overcenter bias means facilitates quick and efficient engagement and disengagement of the tensioning device with the drive belt.